For the butter sage sauce and garnish:
Wash the leaves carefully and cut off any hard stems.
Bring a large pot of water to the boil, add some salt and after that the spinach leaves.
Boil them for a few minutes until they start turning soft (for the spinach about 2 minutes should be enough). Once the leaves have turned soft, drain them in a colander and run cold water over them. Toss the cooked leaves and check if they are cold.
Wait a few minutes allowing the water to drain from the leaves. Now squeeze out very well the remaining liquid with your hands. You can now roughly chop the boiled spinach leaves.
In a large bowl mix the ricotta cheese, chopped spinach, parmesan and one egg. Once the ingredients are well combined season with some nutmeg, salt and pepper to taste.
Prepare the gnudi by taking with a tablespoon the ricotta and spinach mixture and rolling it in your palms into a small ball roughly the size of a walnut. In case the dough is too difficult to manage stir in some flour. I don’t add more than one teaspoon (but in case the spinach leaves were too watery you might need more) and it is normal that the dough will stick a bit to your hands. Help yourself with some flour while rolling the gundi.
Place the prepared gnudi on a floured workspace.
Bring a large pot of salted water to the boil and cook the gundi until they rise to the surface. Scoop them out with a slotted spoon and place them on a serving plate.
In the meantime melt the butter in a pan, add the sage leaves and let the butter foam stirring occasionally. Once the butter becomes lightly golden pour it (with the sage leaves) over the gnudi and sprinkle generously with grated parmesan.